Visual effects

Visual effects — often abbreviated as VFX — are those elements of a shot which cannot be achieved exclusively by practical means during principal photography. They can be accomplished with the varying techniques of CGI, model work and other techniques. Even practical effects can be used as an element in a "visual effects shot", as when specially filmed flame or lava effects are composited into a scene.

For the most part, the starting "canvass" for a VFX is a blue or green screen placed behind actors during principal photography. This solid patch of colour can then be removed and replaced with another image. During much of the original series, this was achieved through a process known as colour separation overlay, whereby a blue screen would be fed images during the live recording of the shot. Since the TV movie, most of the VFX seen on Doctor Who have been achieved through the use of CGI, a more nuanced approach which allows elements rendered on a computer to be composited into a shot captured during principal photography. As contrasted with CSO and other optical/analogue effects, CGI work is completed exclusively in post-production. For this reason, many no longer consider CSO to be a true visual effect, but rather a "special effect".

The term describes both the title the work and the work itself. Thus The Mill provide VFX and were given the title of "Visual Effects" in the credits of Doctor Who from 2005 until 2013. From The Day of the Doctor, Milk filled this role instead.